The invention relates to the field of communications, in particular to wavelength encoded optical communication systems, and more particularly to optical switching means i.e. means for adding, dropping and multiplexing or switching wavelength encoded optical channels.
Optical communications systems are a substantial and fast-growing constituent of communication networks. The expression optical communication system, as used herein, relates to any system that uses optical signals to convey information. Such optical systems include, but are not limited to, telecommunications systems, cable television systems, and local area networks (LANs). Optical systems are described in Gowar, Ed. Optical Communication Systems, Prentice Hall, N.Y. Currently, the majority of optical communication systems are configured to carry an optical channel of a single wavelength over one or more optical waveguides. To convey information from a plurality of sources, time-division multiplexing (TDM) is frequently employed. In time-division multiplexing, a particular time slot is assigned to each signal source, the complete signal from one of the signal sources being reconstructed from the portions of the signals collected from the relevant time slots. While this is a useful technique for carrying information from a plurality of sources on a single channel, its capacity is limited by fibre dispersion and the need to generate high peak power pulses.
While the need for communication services increases, the current capacity of existing waveguiding media is limited. Although capacity may be expanded, e.g. by laying more fibre optic cables, the cost of such expansion is prohibitive. Consequently, there exists a need for a cost-effective way to increase the capacity of existing optical waveguides.
Wavelength division multiplexing (WDM) is now used for increasing the capacity of existing fibre optic networks. In a WDM system a plurality of optical signal channels are carried over a single waveguide, each channel being wavelength encoded i.e. assigned a distinct part of the spectrum. Ideally each channel will be allocated to a wavelength band centered upon a single wavelength. In practice, due to the shortcomings of available sources and spectral broadening due to the modulation on the carrier and due to the dispersion and propagation of transmission media, each signal channel will spread across the spectrum to a greater or lesser extent. References herein to a wavelength are to be interpreted accordingly.
Optical fibre networks have been explored to permit the transfer of optical signals carrying WDM channels (WDM signals) bearing analogue or digital data, from one optical fibre in one loop, ring, cell of a mesh or line of a network to a different loop, ring, cell of a mesh or line of the network directly, in optical form, without the need to convert the signals into electrical form at interconnection points of the network. These interconnection points (or nodes) comprise optical add-drop multiplexers OADMs or optical cross connects OXCs.
Several methods to achieve optical add drop multiplexing (or switching) and optical cross connect switching are described in the proceedings of the European Conference Optical Communications, September, 1998, Madrid, Spain and the Optical Fibre Conference, February 1998, USA.
FIG. 1 shows a known optical add-drop arrangement. The arrangement has four external ports identified by numbers inside rectangles. External port 1 is the Input port for an input signal comprising a set SIN of input channels. This set SIN may be made up from a set ST of through channels and a set SD of drop channels. Port 2 is the Drop port where a drop signal comprising the set SD of drop channels emerges. External port 3 is the Thruxe2x80x2 port for the output of a through signal comprising the set ST of through channels and any add channels SA and port 4 is the Add port for the introduction of an add signal comprising add channels SA. The above channels are wavelength encoded, (e.g. WDM) optical channels.
According to the known arrangement of FIG. 1, switching of one or more selected channels from the set SIN of input channels is achieved by passing the set SIN=ST+SD into a first port Port 1 of a first optical circulator 16. These channels will exit first optical circulator 16 at a second port 17. A series of tunable optical filters represented by rectangles are positioned in optical guide 15 leading from second port 17 of first circulator 16 to a first port 19 on second circulator 18 such that selected ones ST of the set SIN of input channels are passed by the series of filters to second circulator 18, while the other ones SD of the set SIN of input channels are reflected back into second port 17 of first circulator 16 to emerge at a third port thereof, i.e., the Drop port 2, to form the drop signal.
The through channels ST enter second circulator 18 at first port 19 thereof and emerge at Thruxe2x80x2 port 3 forming the through signal. If it is desired to add channels SA to the through signal at Thruxe2x80x2 port 3 i.e. to replace those channels SD dropped as a result of being reflected by one of the filters, these channels SA may be inserted at a third port, i.e. Add Port 4 of second circulator 18 such that they emerge at first port 19 and encounter the series of tunable optical filters positioned between the first and second optical circulators. If the add channels SA are assigned to some of the same wavelengths as the drop channels SD, they will be reflected back into second circulator 18 at first port 19 thereof and will emerge at Thruxe2x80x2 port 3 together with the through channels ST.
In the arrangement of FIG. 1 a filter is required in waveguide 15 leading from the first circulator to the second circulator for each input channel. Switching is achieved by arranging that the filters may be de-tuned or adjusted by an amount comparable to the spectral width of a channel. Hence, if a filter is arranged to normally reflect a particular channel, de-tuning will cause it to pass that channel. Alternatively, the filter may be arranged to normally pass and on de-tuning to reflect a particular channel. This process allows any sub-set of channels to be selected from the total input set SIN.
The performance of the system of FIG. 1 and similar systems will depend on the performance of the filters. Important system performance parameters are insertion loss between ports (i.e. power loss between Ports 1 and 2, 1 and 3, 4 and 3) and the signal rejection (i.e. power coupled into the wrong port such as from Port 1 to Port 3 for channels that have been dropped) and from Port 4 to Port 2 for added channels. The ideal filter response would be zero outside of the desired reflection band and 100% within the band so that when tuned or de-tuned for reflection no power is transmitted through the filter and all is reflected. The rejection and insertion losses would then be limited by imperfections in the optical circulators.
In practice compromises have to be made in the design of a filter. A highly reflective design has high loss outside of the reflection band which increases the insertion loss for other channels passing through it. The more filters used, the greater the insertion loss. A lower reflectivity filter produces less insertion loss for the other channels passing through it but passes more of the optical energy of the channel to be reflected so that the rejection performance becomes poorer. This can cause problems, for example if a new channel is added to the signal passed by the filters. If, as is commonly the case, it is desired to reuse the spectral band occupied by a channel that has been dropped in an OADM (i.e. by reflection by a filter) for adding a new channel, then corruption of the new channel can occur due to optical energy from the dropped channel passed by the lower reflectivity filter.
Thus the design of FIG. 1 will work acceptably for cases where the number of filters in a series is small but the performance deteriorates as the number of filters increases. Hence a problem exists with known OADMs where selected output channels contain unwanted residual elements from other channels. Known designs of optical switch also use series of selectively reflective optical filters as selective reflectors. The same problem is encountered with these known designs of optical switch in that selected output channels contain unwanted elements from other channels.
The present invention provides an optical switching means for receiving an input signal comprising a plurality of input wavelength encoded optical channels the switching means comprising selection means for selecting from the plurality of input optical channels for forming a plurality of output signals comprising selected ones of the plurality of input optical channels in which the combination of input optical channels in any one of the plurality of output signals is different from the combination of input optical channels in the input signal; in which the selection means comprises a plurality of selective reflectors for selectively reflecting optical channels selected from the plurality of input optical channels, and in which the plurality of selective reflectors are arranged to selectively reflect each of the input optical channels selected for forming the plurality of output signals.
The present invention also provides an optical switching means comprising a plurality of ports, the switching means for forming at the plurality of ports from a plurality of wavelength encoded input optical channels a plurality of output optical signals comprising wavelength encoded output optical channels in which the switching means comprises selective reflectors each selective reflector for reflecting a first portion of a selected input optical channel to form an output channel and for passing a residual portion of the selected input optical channel in which the switching means comprises separation means for separating the residual portions from the output signals.
The present invention also provides an optical switching means for receiving an input signal comprising a plurality of input wavelength encoded optical channels the switching means comprising selection means for selecting from the plurality of input optical channels for forming a plurality of output signals comprising selected ones of the plurality of input optical channels, in which the selection means comprises a plurality of selective reflectors for selectively reflecting optical channels selected from the plurality of input optical channels, in which the plurality of selective reflectors comprise a first set of selective reflectors for passing a first set of selected optical input channels and for reflecting a second set of selected optical input channels and a second set of selective reflectors for reflecting the first set of selected optical input channels and for passing the second set of selected optical input channels.
In a preferred embodiment, the present invention provides an optical switch comprising the above optical switching means.
In a further preferred embodiment, the present invention provides an optical add-drop multiplexer comprising the above optical switching means.